Which of these numbers is composite? ${13,\ 31,\ 37,\ 47,\ 63}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 13, 31, 37, and 47 each have only two factors. The factors of 63 are 1, 3, 7, 9, 21, and 63. Thus, 63 is the composite number.